I interviewed at both schools. Eventually I got into my top choice so I'm withdrawing from both.
What does your financial aid look like? Dartmouth has great financial aid. Pitt seemed...not so great in this department. For me, research and overall 'prestige' goes to Pitt. Environment and happiness went to Dartmouth hands down. I think you could get a global health thing going at either school. Campus and area totally went to Dartmouth. Feel of the school was also in Dartmouth's favor.
If I was accepted to both with a equal financial burden, I would totally go with Dartmouth. Which is something to consider if you are this torn. All things being equal - follow the money.
Edit: Don't stress yourself out over this - you are accepted to two great schools. It is good to have such a dilemma.